A presensitized lithographic plate usually comprises a support, a hydrophilic layer and a light-sensitive layer in that order.
As computer technology has been recently developed, a great technical innovation has been introduced into printing. The field of color printing is now digitized by using a computer system called CEPS (Color Electronic Prepress System). The CEPS digitizes the printing data of a color image and the data of all the prepress process. Further, the newest system directly outputs the digitized printing data to a presensitized lithographic plate of a high sensitivity to prepare a lithographic plate without use of a conventional lith film. The lithographic plate for the system is called DDPP (Digital Direct Printing Plate).
A presensitized DDPP is exposed to light corresponding to the digitized data. The exposure step is preferably conducted in commercially available printing machines such as an image setter or a scanner (e.g., SG757 and SG608 of Dai Nippon Printing Co., Ltd.; Select Set.TM. and Accuset.TM. of Agfa; Lino.TM. 300 of Lino; and Magna.TM. 646 of Crossfield). In the image setter or the scanner, a flexible.(e.g., plastic or paper) support of a plate is preferred to a hard (e.g., aluminum) support.
According to a lithographic process, the unhardened (unexposed) area of a light-sensitive layer is removed from the support. A hydrophilic layer is arranged between the support and the light-sensitive layer to form an ink-repulsive surface. The hydrophilic layer usually contains solid particles dispersed in a hydrophilic binder. The solid particles form a rough surface on the hydrophilic undercoating layer. The rough surface can contain dampening water when the plate is used in printing.
The components of a light-sensitive layer are determined depending on the lithographic method. For example, a light-sensitive layer for a photopolymerization method may contain a photopolymerization initiator and an ethylenically unsaturated polymerizable compound or a cross-linkable polymer, as is described in U.S. Pat. Nos. 2,927,022, 2,902,356 and 3,870,524. Further, the light-sensitive layer may comprise a silver halide emulsion layer and a physical development nucleation layer according to a silver complex salt diffusion transfer method. A presensitized plate for the silver complex salt diffusion transfer method is commercially available (e.g., Silver Digiplate.TM. of Mitsubishi Paper Mills Co., Ltd.; and Set Print.TM. of Agfa).